Molecular evolution of the VacA p55 binding domain of Helicobacter pylori in mestizos from a high gastric cancer region of Colombia

Introduction

H. pylori is a highly successful Gram-negative bacterium that colonizes the stomach of about 50% of the human world population (Perez-Perez, Rothenbacher & Brenner, 2004; Khalifa, Sharaf & Aziz, 2010). The infection is associated with superficial gastritis; however, a subset of individuals can develop ulcers (Ernst & Gold, 2000) or two aggressive forms of cancer, mucosa-associated lymphoid tissue (MALT) lymphoma or gastric adenocarcinoma (Parsonnet et al., 1991; Parsonnet et al., 1994; Blaser, 1998; Posselt, Backert & Wessler, 2013). Thus, H. pylori infection is recognized as the most important risk factor for gastric cancer development (International Agency for Research on Cancer (IARC), 1994; Helicobacter and Cancer Collaborative Group, 2001). In Colombia, gastric cancer is the fourth cause of deaths by cancer (Ferlay et al., 2013) and the risk to develop this disease increases following the altitudinal gradient (Torres et al., 2013). In this country, an inverse relation in gastric cancer risk has been observed between inhabitants from the Andes with high risk and those located at the coasts with low risk (Kodaman et al., 2014). Bogotá and its surrounding towns, the places from where the clinical isolates of the present study were obtained, are located in the Andean mountain and consequently have a high gastric cancer risk (Marion & Murillo, 2004) and its population is conformed predominantly by mestizos (Ossa et al., 2016; Adhikari et al., 2016).

H. pylori and humans have co-evolved for at least 100,000 years, and the bacteria have mimicked the settlement pattern of its host (Linz et al., 2000). Seven major populations of H. pylori have been discovered on our planet: hpEurope, hpNEAfrica, hpAfrica1, hpAfrica2, hpAsia2, hpSahul and hpEastAsia (Yamaoka, 2009; Falush et al., 2003; Achtman, 2007; Achtman et al., 1999; Moodley et al., 2009; Devi et al., 2007). About 12,000 to 15,000 years ago, humans carrying the hpEastAsia H. pylori population migrated to the American continent by crossing the Bering Strait, which developed towards a new hspAmerind cluster (Kersulyte et al., 2010). However, the Spanish colonization era, starting in the 15th century, changed the load of pathogens in the native human population of Latin America (Bianchine & Russo, 1992). Thus, new strains of H. pylori were introduced to the continent by European conquerors and also African slaves, which, through recombination, gene conversion and natural selection, produced unique evolutionary lineages of the bacterium in the mestizos of Colombia and other countries of Latin America (Gutiérrez-Escobar et al., 2017; Thorell et al., 2017).

One of the most studied virulence factors of H. pylori is the vacuolating cytotoxin VacA (Leunk et al., 1988; Chung et al., 2010; Backert & Tegtmeyer, 2010; Jang et al., 2010). This toxin is present in all H. pylori strains and does not have homologues in other bacterial species (Cover & Blanke, 2005). VacA displays two main cellular localizations: on the bacterial cell surface (Ilver et al., 2004) or as a secreted toxin of about 88 kDa (Cover & Blaser, 1992). The secreted toxin is cleaved into two smaller products, called p33 and p55. While p55 binds to the host cell and has been used to classify the m1 or m2 allelic VacA types, p33 and part of p55 are responsible for the intracellular vacuolating activity (Ye, Willhite & Blanke, 1999; Torres, McClain & Cover, 2004; Ji et al., 2000). Previous phylogenetic analysis using the full-length VacA sequence revealed three discrete phylogenetic clusters, one for non-Asian strains, other for exclusively Asiatic strains and the last conformed by a worldwide mixture, the first two were assigned to the type m1; meanwhile the last to the type m2 (Gangwer et al., 2010). In the same study, they also showed that the phylogeny of the p55 domain resembles the phylogeography of full-length VacA and found that adaptive evolution has driven its divergence patterns (Gangwer et al., 2010).

A recent study has also shown that the evolution of VacA in Latin America has been strongly biased due to the effects of Spanish colonization (Gutiérrez-Escobar et al., 2017). However, neither the phylogeny nor the way in which natural selection has operated on the p55 domain has been described yet in H. pylori isolates from this region. In this context, the aim of the present study was to describe the genetic diversity and microevolution of the p55 domain in a large group of clinical H. pylori isolates obtained from mestizos in a high gastric cancer risk zone from Colombia.

Material and Methods

The DNA and protein sequences corresponding to the p55 domain of VacA were obtained from 101 genomes previously sequenced by our group from the H. pylori stock collection at the Instituto Nacional de Cancerología in Bogotá (Gutiérrez-Escobar et al., 2017). The stock collection was obtained from inhabitants of Bogota, Tunja and surrounding towns. The study region is located in a high plateau (8,660 ft), also called the Bogota savanna, and is part of the Altiplano Cundiboyacense at the Eastern Cordillera of the Andes. This region is characterized by its high gastric cancer risk (International Agency for Research on Cancer (IARC), 1994; Helicobacter and Cancer Collaborative Group, 2001). The sequences were categorised in four groups according to the gastric disease state as follows: 31 cases of gastritis (G), 17 cases of gastric adenocarcinoma (GA), 27 cases of atrophic gastritis (AG), and 26 cases of intestinal metaplasia (IM). For the m region characterization, sequences corresponding to m1, m2 and m1/m2 types were downloaded from Genbank. For m1: Q48245; chimeras (ch): Q9 kJA6, Q6DLS8; and m2: Q48253 (Cover et al., 1994; Atherton et al., 1999. The sequences were aligned using MUSCLE software (Edgar, 2004); and displayed in ESPript http://espript.ibcp.fr (Robert & Gouet, 2014).

Results

In our previous analysis of genetic differentiation of the vacA gene using 101 H. pylori genome sequences of mestizos from a high gastric cancer region in the Andes of Colombia and 34 reference genomes showed that the Colombian alleles are more similar to that of HpEurope or HspWestAfrica, differentiating them from HspAsia and HspAmerind strains (Gutiérrez-Escobar et al., 2017). This indicates that the Colombian population may have a remarkable European-African component with regard to the VacA type, which was not yet studied. In the present study, we performed a molecular evolution analysis of the p55 domain of VacA from the 101 H. pylori isolates (Table S1). For this purpose, the p55 protein sequences were aligned against four reference sequences belonging to the m1, m2 and chimera allele types. The alignment revealed that 84% of the sequences from Colombian isolates belonged to the m1 type, whereas 15.2% belonged to the m2 type (Fig. 1).

The vacA gene fragment that encodes for the p55 domain in Colombian clinical isolates showed 431 segregating sites and 95 haplotypes with a haplotype diversity of 0.999. The nucleotide diversity (π) was 0.0741 and the average number of nucleotide differences (k) was 99.523. The Tajima’s D test was 0.669, indicating that rare alleles are at low frequency in the population. The phylogenetic tree of p55 VacA showed three major clades corresponding to the m1, m2 and chimera types. Although the phylogenetic tree has very short branches indicating a very recent evolutionary history, seven independent lines were identified for the m1 sequences, one for chimeras and one for m2 sequences, suggesting that diversifying selection has influenced the evolution of this domain in the high gastric risk zone in Colombia (Fig. 2). Recombination and gene conversion played an important role in the evolution of p55 vacA gene sequences. The Betran method showed 24 gene conversion tracts for the entire sample. However, when the sequences were analyzed per disease group, all shared genetic tracts. The Hudson R estimator showed 98 minimal recombination events varying from the AG population with 69 events until the GA with 32 (Table 1).

As next we aimed to investigate p55 at the amino acid level. For this purpose, the p55 protein model of the Colombian sequence 1077GA was obtained by using the published crystal structure (PDB code 2QV3) as a template, which showed a high structural similarity (Gangwer et al., 2007). The N- and C-termini of the p55 VacA model showed the typical β-strands and α-helices previously identified in the 2QV3 crystal (Gangwer et al., 2007). Likewise, the RMSD predicted for the above p55 VacA model was 0.198 Å for alpha carbon, and 0.61 Å  for the backbone. About 83% of the amino acid residues were surface-exposed and 17% were classified as buried inside the protein (Figs. 3A/ 3B).

The clades previously identified in the phylogenetic tree were used to detect functional divergence events amongst the p55 VacA sequences. Although, the pairwise comparisons between the clades 1 and 2 showed that the sites at amino acids P454 and L342 have a θD>0.5, this result was not significant (p > 0.07). The dataset was corrected for recombination using the Single Break Point (SBP) algorithm and then positive and negative selection analyses were performed. The analysis showed that according to the FEL test, 3.7% residues evolved under positive selection and 22.2% by purifying selection, and the IFEL test indicated that 3.7% evolved by positive selection and 17.3% under purifying selection (Table 2 and Table S2 to clarify the numbering system). Likewise, 8.7% of p55 VacA residues showed episodic diversifying selection according to the MEME test (Table 3 and Table S2).

To identify if immune selection also played a role in the evolution of p55 VacA, an epitope prediction test was performed. According to this prediction, the 48.3% of p55 VacA has the potential to be recognized by humoral immunity (Fig. 4A). All the positive and episodic diversifying selected sites detected using the FEL, IFEL and MEME tests were mapped against the p55 VacA structure. About 58.8% of the positive selected sites according to the FEL and IFEL test and the 35% of the episodic diversified selected sites according to MEME test were located in the regions predicted to be recognized by the host humoral immunity. This result implies that immune selection may have contributed to shaping the evolutionary pattern of this protein domain in strains from Colombia (Figs. 4B/4C). Besides, the amino acid residues R321 and V261 (see Table S2), both recognized by FEL and IFEL test under positive selection, showed significant variation between all possible pairwise combinations of the p55 VacA groups when the Pearson chi-squared test was performed (Table 4).

Discussion

Gene diversification and duplication are important sources of biological innovation during evolution (Zhang, 2003). For example, duplicated genes evolve in two pathways; they either become functional novelties or become functionless (Lynch & Conery, 2000). It has been shown that gene duplication and frameshift mutations have an important role in vacA evolution, not only in Helicobacter pylori but also in other species of this genus (Ito et al., 1998; Foegeding et al., 2016). Likewise, secreted VacA recognizes several host cell receptors; for example, the proteins LRP1 (Yahiro et al., 2012), RPTP (Yahiro et al., 2003; Yahiro et al., 2004) and EGFR (Seto et al., 1998; Tegtmeyer et al., 2009) as well as sphingomyelin (Utt, Danielsson & Wadstrom, 2001; Gupta et al., 2008), glycosphingolipids (Gupta, Wilson & Blanke, 2010), heparan sulphate (Roche et al., 2007) and phospholipids (Molinari et al., 1998), indicating that functional novelty may have occurred during evolution.

The genetic structure of p55 VacA from Colombian isolates indicates that some alleles evolves under positive selection according to Tajima’s D (D = 0.669), and the high number of haplotypes suggests a low frequency of rare alleles. It is important to stress that the observed nucleotide diversity is low, indicating that a population contraction could have taken place in this region from Colombia, but advantageous variants of p55 VacA were maintained through balancing selection. Several studies have suggested that Latin American mestizos have an admixture of ancestries from Europe and Africa (Falush et al., 2003; Thorell et al., 2016). After colonization by the European conquerors, H. pylori may evolved alongside its mestizo host producing independent evolutionary lines as shown by multilocus sequence typing (MLST), phylogenomic and AlpA adhesin analyses using genomes sequences of the same isolates from which we obtained the p55 sequences (Gutiérrez-Escobar et al., 2017; Gutiérrez-Escobar et al., 2018). This could explain a possible population contraction and the dominant purifying selection detected by analysis of the p55 VacA domain.

The phylogenetic tree of p55 VacA showed three principal clades –one for the m1 type and the others for the m2 and chimeras in Colombian mestizos, respectively, suggesting a possible functional divergence event. The phylogenetic tree showed very short terminal branches, which indicates that mutations were accumulated over a short period of time and that a bottleneck process took place in the region, where only few organisms survived a strong selective pressure that reduced the population. When the branch distances were ignored, the phylogenetic tree showed multiple points of functional diversification between the p55 VacA variants. This diversification could represent functional adaptation to host cell receptors (Gangwer et al., 2010) and/or immune selective pressure ( Ghose et al., 2007).

A previous study showed that the divergence between vacA alleles is due to positively selected surface-exposed sites in the p55 cell binding domain (Gangwer et al., 2010). In this study, the different algorithms also detect positive selected sites in the surface of the protein, but the major contribution to the microevolutionary patterns observed for p55 VacA was made by the strong purifying selection. The positive selected sites detected were defined principally as episodic diversifying residues supporting the observed branching scheme of the phylogenetic tree.

Another important force that shaped the microevolution of p55 VacA was recombination and gene conversion. Helicobacter pylori can take-up exogenous DNA from the environment (Nedenskov-Sørensen, Bukholm & Bøvre, 1990) and recombine it with an extremely high frequency (Go et al., 1996; Suerbaum et al., 1998). The number of recombination events detected in Colombian p55 VacA suggests that the strains exchanged DNA following a free recombination pattern. This indicates that, in addition to the low nucleotide diversity, the population exhibits a high level of genetic homogeneity. Perhaps, after the initial Spanish colonization, a small initial population of new H. pylori subtypes exchanged genetic information by recombination and gene conversion, producing a highly homogeneous strain pool in this country. It has been shown that recombination is more efficient between related strains than unrelated ones (Israel, Lou & Blaser, 2000; Pinto et al., 2005).

The entire positive selected sites and most of the purifying selected ones were located at positions with a high likelihood to be recognized by the host immune system. We propose that immune selection has triggered the diversification of Colombian p55 VacA, but at the same time those diversified alleles have exacerbated the host immune response contributing to the high prevalence of gastric diseases and gastric cancer observed in this geographic area as analysed by the Red Queen model of evolution (Strotz et al., 2018). One of the most important results presented here is the detection of a plausible relation between natural selection and the gastric disease state. Two specific amino acid positions, R261 and L321 according to the reference p55 VacA (Gangwer et al., 2007), both under positive episodic diversifying selection in the Colombian samples revealed significant sequence variations between different disease state groups, which opens new opportunities for the development of early diagnosis strategies specifically addresses to this region from Colombia.

Conclusions

Taken together, Latin-America represents an “evolutionary laboratory” for H. pylori, and it is possible that a new variant of virulence factors such as VacA has been evolving rapidly in this subcontinent. This rapid evolutionary process has been described for example on the AlpA adhesin in the same region (Gutiérrez-Escobar et al., 2018). We assume that 500 years of colonization provided sufficient time to produce new allelic variants for p55 VacA not only in Colombia, and possibly also in other countries of the region. Further studies should investigate this in more detail in H. pylori isolates across Latin America.

Supplemental Information